This is so good!
Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan with cooking spray.
Place chicken breast halves between 2 sheets of heavy plastic on a solid, level surface. Firmly pound with the smooth side of a meat mallet to 1/4-inch thickness.
Mix ricotta cheese, spinach, sun-dried tomatoes, and Parmesan cheese together in a bowl.
Place 1 slice prosciutto on top of each chicken breast; spread ricotta cheese evenly on top. Roll up chicken and secure with a toothpick.
Arrange chicken rolls in the prepared baking pan; cover with 1 cup marinara sauce. Cover baking pan with aluminum foil.
Bake in the preheated oven until sauce is bubbly, about 15 minutes. Uncover and baste chicken rolls. Continue baking, uncovered, until chicken is no longer pink in the center, about 5 minutes more.
Slice each chicken roll into 4 or 5 slices. Spoon remaining 3/4 cup marinara sauce on top.